Automatic transmissions for transmitting power between an input and an output, either over a continuously variable range of speed ratios or in discrete step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, open hydraulic friction clutches and brakes, hydraulic pump, and gear meshes.
To improve fuel economy in a motor vehicle having an automatic transmission, an automated shift manual (ASM) transmission can be used to eliminate or substantially reduce all of these parasitic losses except gear mesh losses. An ASM transmissions, are limited in the rate at which they can dissipate the excess power. The amount transmission generally performs gear ratio changes by first interrupting torque transmitted from the engine to the transmission input, preparing the transmission components associated with the next speed ratio, and then restoring torque at the input. A primary functional feature of ASM transmissions is the need to interrupt power transmitted from the engine to the transmission input shaft before or during each gear ratio change.
Dual clutch layshaft transmissions are essentially two ASM transmissions, one providing odd numbered gears and one providing even numbered gears. Shifts between odd numbered gears and even numbered gears can be accomplished without interrupting power flow. While operating in an odd numbered gear, couplers can be actuated to configure the transmission for the next even numbered gear. Dual clutch transmissions have parasitic losses only slightly higher than ASM transmissions.
When a motor vehicle is accelerated from rest, the mechanical power generated by the engine exceeds the power utilized by the vehicle. The transmission must dissipate the difference, generally as heat. Open torque converters are very efficient at converting the excess mechanical power into heat in the working fluid. Friction clutches, as used in ASM and dual clutch of energy that must be dissipated is determined by the torque level, the speed difference across the clutch, and the duration of the event.
The most effective way to limit the power that must be dissipated by the clutch is to provide additional torque multiplication in the gearbox. This has two benefits. First, it reduces the torque which the clutch must transmit. Second, it reduces the duration of the event because the gearbox input will become equal to the engine speed at a lower vehicle speed. The need for similar top gear ratios, which is dictated by cruising fuel economy, is unchanged, so the resulting gearbox must have substantially more total span. The difference between adjacent gear ratios is limited by the ability to make comfortable shifts. As a result, it is also necessary to increase the number of discrete gear ratios.
Traditionally, one reverse ratio has been considered sufficient, since speed is relatively low and fuel efficiency in reverse is not a significant concern. However, if the gear multiplication is high enough to satisfy clutch thermal considerations, it may be excessive for normal reverse driving, even at those relatively low speeds. Therefore, it is beneficial to provide a reverse ratio similar to the traditional reverse ratio in addition to one that has much more multiplication.
One known way to increase the gear multiplication is to increase to ratio of the tooth counts for individual gear pairs. This would require increasing the distance between shafts due to limitations on how small the gears can be relative to the shaft diameter. Adding an additional forward and reverse ratio would ordinarily require at least four additional gears and an additional synchronizer sleeve. The resulting transmission would be much larger and likely would not fit into the package space available.
In a layshaft transmission, gears connected to a drive path moving sleeves on a coupler, such as a synchronizer. In a dual clutch transmission (DCT), one or two gears may be selected at any time, provided each gear is associated with a different input clutch. The clutch associated with the gear being selected or deselected must be disengaged while the coupler sleeve is moved.
In a DCT that includes a clutch coupler, operation in the lowest forward gear, first gear, requires that two synchronizers to be engaged: a second gear coupler and the clutch coupler. Similarly, operation in the lowest reverse gear, R1 gear, requires that a R2 coupler and the clutch coupler be engaged.
The sequence in which these couplers are engaged greatly influences the magnitude of coupler torque required. For example, if second gear is engaged first, the second gear coupler must accelerate only one clutch disc, and it has a moderate torque ratio to that clutch disc. On the other hand, if the clutch coupler is engaged first, subsequent engagement of the second gear coupler requires accelerating both clutch discs concurrently while overcoming a much larger torque ratio. This is particularly troublesome if the clutches are very cold, which causes them to have high drag.
There is a need in the automotive industry for a gear shift control strategy that ensures engagement of the clutch coupler is performed last.
Rock cycling the vehicle by moving the gear selector between the R-range and D-range is commonly used to move the wheels from snow, ice or mud. Preferably, the gear shift control strategy would only switch clutches and not move any coupler sleeves to switch from a forward ratio to a reverse ratio during rock cycling operation.